Battery plate containing filler with conductive coating

ABSTRACT

The plate (10) comprises a matrix or binder resin phase (12) in which is dispersed particulate, conductive tin oxide such as tin oxide coated glass fibers (14). A monopolar plate (11) is prepared by coating a layer (18) of electrolytically active material onto a surface of the plate (10). Tin oxide is prevented from reduction by coating a surface of the plate (10) with a conductive, impervious layer resistant to reduction such as a thin film (22) of lead adhered to the plate with a layer (21) of conductive adhesive. The plate (10) can be formed by casting a molten dispersion from mixer (36) onto a sheet (30) of lead foil or by passing an assembly of a sheet (41) of resin, a sheet (43) of fiberglass and a sheet (45) of lead between the nip of heated rollers (48, 50).

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 83-568 (72 Stat435; 42 USC 2457).

This is a division of application Ser. No. 550,875, filed Nov. 14, 1983,now U.S. Pat. No. 4,510,219.

TECHNICAL FIELD

The present invention relates to secondary batteries of the bipolarplate type and, more particularly, to an improved lightweight batteryplate for use in fabricating bipolar or monopolar plates for lead-acidbatteries.

BACKGROUND ART

Even though there has been considerable study of alternativeelectrochemical systems, the lead-acid battery is still thebattery-of-choice for general purpose uses such as starting a vehicle,boat or airplane engine, emergency lighting, electric vehicle motivepower, energy buffer storage for solar-electric energy, and fieldhardware whether industrial or military. These batteries may beperiodically charged from a generator.

The conventional lead-acid battery is a multicell structure. Each cellcontains a plurality of vertical positive and negative plates formed oflead-based alloy grids containing layers of electrochemically activepastes. The paste on the positive plate when charged contains leaddioxide which is the positive active material and the negative platescontain a negative active material such as sponge lead. This battery hasbeen widely used in the automotive industry for many years, and there issubstantial experience and tooling in place for manufacturing thisbattery and its components, and the battery is based on readilyavailable materials, is inexpensive to manufacture and is widelyaccepted by consumers.

The open circuit potential developed between each positive and negativeplate is about 2 volts. Since the plates are connected in parallel, thecombined potential for each cell will also be about 2 volts regardlessof the number of plates utilized in the cell. One or more cells are thenconnected in series to provide a battery of desired voltage. Common lowvoltage batteries of 6 volts have 3 serially connected cells, 12 voltbatteries include 6 serially connected cells and 24 volt batteriescontain 12 serially connected cells. The bus bars and top straps usedfor intercell connection add to the weight and the cost of the batteryand often are subject to atmospheric or electrochemical corrosion at ornear the terminals.

Another problem with lead-acid batteries is their limited lifetime dueto shedding of the active materials from the vertically orientedpositive and negative plates. During operation, these electrodematerials shed and flake and fall down between the vertically orientedplates and accumulate at the bottom of the battery. After a period ofoperation, sufficient flakes accumulate to short circuit the gridsresulting in a dead battery cell and shortened battery life.

Lead-acid batteries are inherently heavy due to use of the heavy metallead in constructing the plates. Modern attempts to produce lightweightlead-acid batteries, especially in the aircraft, electric car andvehicle fields, have placed their emphasis on producing thinner platesfrom lighter weight materials used in place of and in combination withlead. The thinner plates allow the use of more plates for a givenvolume, thus increasing the power density. Some of these attempts haveincluded battery structures in which the plates are stacked inhorizontal configurations. Higher voltages are provided in a bipolarbattery including bipolar plates capable of through-plate conduction toserially connect electrodes or cells. The horizontal orientation of thegrids prevents the accumulation of flake lead compounds at the batterybottom. Downward movement of electrolyte can be blocked by use of glassor porous polypropylene mats to contain the electrolyte. Also,stratification of electrolyte is prevented since the electrolyte isconfined and contained within the acid resistant mats by capillaryaction.

The bipolar plates must be impervious to electrolyte and be electricallyconductive to provide a serial connection between cells. The bipolarplates also provide a continuous surface to prevent loss of activematerials.

Most prior bipolar plates have utilized metallic substrates such as leador lead alloys. The use of lead alloys, such as lead antimony, givesstrength to the substrate but causes increased corrosion and gassing.

Alternate approaches have included plates formed by dispersingconductive particles or filaments such as carbon, graphite or metal in aresin binder such as polystyrene (U.S. Pat. No. 3,202,545), a plasticframe of polyvinyl chloride with openings carrying a battery activepaste mixed with nonconductive fibers and short noncontacting leadfibers for strengthening the substrate (U.S. Pat. No. 3,466,193), abiplate having a layer of zinc and a polyisobutylene mixed withacetylene black and graphite particles for conductivity of the plate(U.S. Pat. No. 3,565,694), a substrate for a bipolar plate includingpolymeric material and vermicular expanded graphite (U.S. Pat. No.3,573,122), a rigid polymer plastic frame having a grid entirely of leadfilled with battery paste (U.S. Pat. No. 3,738,871), a thin, plasticsubstrate having lead strips on opposite faces, the lead strips beinginterconnected through an opening in the substrate and retained byplastic retention strips (U.S. Pat. No. 3,819,412), and a biplate havinga substrate of thermoplastic material filled with finely dividedvitreous carbon and a layer of lead-antimony foil bonded to thesubstrate for adhering active materials (U.S. Pat. No. 4,098,967).

Some more recent examples of batteries containing bipolar plates areU.S. Pat. No. 4,275,130 in which the biplate construction comprises athin composite of randomly oriented conductive graphite, carbon or metalfibers imbedded in a resin matrix with strips of lead plated on oppositesurfaces thereof. Ser. No. 279,841, filed July 2, 1981, discloses abiplate formed of a thin sheet of titanium covered with a conductive,protective layer of epoxy resin containing graphite powder.

Dispersed, conductive fibers form a conduction path by point-to-pointcontact of particles or fibers dispersed in an insulating matrix resin,and the through-plate serial conductivity is usually lower than desired.Fibrous fillers do increase the strength of the plate by forming afiber-reinforced composite.

It has been attempted to increase the conductivity and strength ofbipolar plates by adding a conductive filler such as graphite. Graphitehas been used successfully as a conductive filler in otherelectrochemical cells, such as in the manganese dioxide, positive activepaste of the common carbon-zinc cell, and it has been mixed with sulfurin sodium-sulfur cells. However, even though graphite is usually afairly inert material, it is oxidized in the agressive electrochemicalenvironment of the lead-acid cell to acetic acid. The acetate ionscombine with the lead ion to form lead acetate, a weak salt readilysoluble in the sulfuric acid electrolyte. This reaction depletes theactive material from the paste and ties up the lead as a salt which doesnot contribute to production or storage or electricity. Acetic acid alsoattacks the lead grids of the positive electrodes during charge,ultimately causing them to disintegrate. Highly conductive metals suchas copper or silver are not capable of withstanding the high potentialand strong acid environment present at the positive plate of a lead-acidbattery. A few electrochemically-inert metals such as platinum arereasonably stable. But the scarcity and high cost of such metals preventtheir use in high volume commercial applications such as the lead-acidbattery. Platinum would be a poor choice even if it could be afforded,because of its low gassing overpotentials.

A low cost, lightweight, stable bipolar plate is disclosed in mycopending application Ser. No. 346,414, filed Feb. 18, 1982, for BIPOLARBATTERY PLATE. The plate is produced by placing lead pellets intoapertures formed in a thermoplastic sheet and rolling or pressing thesheet with a heated platen to compress the pellets and seal them intothe sheet. This method involves several mechanical operations andrequires that every aperture be filled with a pellet before heating andpressing in order to form a fluid-impervious plate.

DISCLOSURE OF THE INVENTION

An improved, lightweight conductive plate for a lead-acid battery isprovided by the present invention. The plate is resistant to theelectrochemical environment of the cell. The plate is prepared in asimple, reliable manner to form a low-resistance, fluid-impervious,through-conductive plate.

The conductive plate of the invention contains a dispersion in a matrixresin of a conductivity additive that is thermodynamically stable to theelectrochemical environment of the lead-acid cell, both with respect tothe strong sulfuric acid electrolyte and to species generated underoxidation and reduction conditions experienced during charge anddischarge of the battery.

A preferred conductivity additive for the plate of the present inventionis conductive tin dioxide (SnO₂). SnO₂ can be present as a powder orcoated onto a particulate or fibrous substrate such as glass powder orglass wool as disclosed in my copending application Ser. No. 488,199,filed April 25, 1983, entitled IMPROVED POSITIVE BATTERY PLATE, thedisclosure of which is expressly incorporated herein by reference.Stannic oxide has a conductivity several times that of graphite. SnO₂(doped) has a conductivity of 300 to 400 micro ohm-cm vs. 1373 microohm-cm for graphite.

Stannic oxide is thermodynamically stable to the oxidation/reductionpotential in the electrochemical environment of the positive plate of aleadacid battery, has about the same resistivity as PbO₂ when SnO₂ isdoped with a suitable dopant such as fluoride ion, and refractory orbaked type of SnO₂ is quite insoluble in sulfuric acid. The stannicoxide conductivity additive will remain unchanged during the course ofcharge and discharge of the battery.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a bipolar plate in accordance with theinvention;

FIG. 2 is a view in section of a monopolar plate prepared in accordancewith the invention;

FIG. 3 is a schematic view of an apparatus for forming a bipolar platein accordance with the invention;

FIG. 4 is a schematic view of an alternate method for forming a bipolarplate in accordance with the invention;

FIG. 5 is a section taken on line 5--5 of FIG. 4; and

FIG. 6 is a view in section of a stack of planar plates forming abattery cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the plate 10 is formed of a composite of anorganic synthetic resin 12 in which is dispersed a sufficient amount ofa stable filler 14 to provide through-plate conductivity. The preferredmaterial is tin oxide in particulate form, preferably coated onto aparticulate support such as glass fibers or glass particles. The glassfibers can be in roving, chopped or glass wool form. In one embodiment,the glass particles are preferably sintered into a solid sheet having aporosity from about 60 to about 90 percent. The plate 10 preferably hasa thickness from about 1 to about 20 mils, more preferably from about 4to about 10 mils.

The coating of stannic oxide onto glass to form a conductive coating wasdeveloped over 30 years ago and has been widely practiced to defrostwindshields in aircraft and automobiles. The conductive coating isapplied to heated glass fibers or powder or glass wool from a solutionof stannic chloride in hydrochloric acid as disclosed in U.S. Pat. No.2,564,707, the disclosure of which is expressly incorporated herein byreference. The solution can be sprayed onto the heated fibers

The diameter of the glass fibers is preferably very small such as fromabout 1 to about 20 microns. Very fine fibers are too hard to handle andlarge diameter fibers have too small a surface to provide adequateconductive surface. The fibers preferably contain a conductive coatingof stannic oxide ranging in thickness from a monolayer up to about 10microns, more preferably from 0.2 micron to 2 microns.

Referring now to FIG. 2, the through-conductive plate 10 can be used asthe central substrate to form a monolayer plate 11 such as a positiveplate containing a layer 18 of positive active material such as leadoxide paste.

Referring back to FIG. 1, since tin oxide is not stable in the reducingenvironment of a negative electrode, the surface 15 facing the negativeelectrode must contain a layer 20 that is conductive and stable underreducing conditions. The layer 20 can be a composite of a syntheticorganic resin such as epoxy or polypropylene containing containing adispersion of about 20 to about 80 percent by weight of conductivefibers which are stable under reducing conditions such as graphitefibers or lead fibers. The layer 20 can also be a thin film or foil oflead. The layer 20 can be adhered to the plate 10 by a film 21 ofconductive adhesive. The fabrication of the bipolar plate is completedby depositing a laying 22 of negative active material such as lead pasteonto the layer 20.

The synthetic organic resin 12 can be thermoplastic. Preferredthermoplastics are the polyolefins such as polypropylene.

The conductive plate of the invention can be readily fabricated bycasting or roll molding techniques. Referring now to FIG. 3, the plateis fabricated by placing a sheet 30 of lead foil on the bottom surface32 of the casting cavity 34. A mixture of molten resin containing atleast 20 to 80 percent by weight of tin oxide coated glas fibers is thenpoured from the mixing kettle 36 into the casting cavity 34. After theresin cools, a conductive layer 38 attached to the lead foil 30 isformed.

Referring now to FIG. 4, another apparatus for forming a conductiveplate includes a supply roll 40 of a thermoplastic resin 41 such aspolypropylene, a supply roll 42 of tin oxide coated fiberglass fabric 43and a supply roll 44 of lead foil 45 having an upper surface coated witha layer 46 of a heat curable, conductive adhesive such as an epoxyfilled with graphite fibers and/or powder. The sheet 43 of fiberglasshas a thickness slightly less than that of the sheet 41 ofpolypropylene. When the three sheets are drawn through heated rollers48, 50, the polypropylene 41 softens. The fabric is pressed onto thesoftened resin to form a composite layer 49 and also attaches the foil45 to form the assembly as shown in FIG. 5.

The following experiments were conducted to evaluate the performance ofthin films of stannic oxide in the environment of a lead-acid battery.

EXAMPLE 1

Glass plates were coated with a conductive coating of stannic oxidefollowing the procedure of U.S. Pat. No. 3,564,707.

EXAMPLE 2

The stannic oxide coated glass plates of Example 1 were immersed in 5.3M H₂ SO₄ at both 20° C. and 50° C. The plates were withdrawnperiodically and the resistance of the thin film coating was measured.The results of measurements during 33 days are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Chemical corrosion of stannic oxide thin film                                 in 5.301 M H.sub.2 SO.sub.4.                                                           50° C. ELECTRODE                                                                      20° C. ELECTRODE                               TIME     RESISTANCE     RESISTANCE                                            (DAYS)   Ω (20° C.)                                                                      Ω (20° C.)                               ______________________________________                                         0       10.95          10.84                                                  1       10.94          10.84                                                  8       10.95          10.84                                                 16       10.94          10.84                                                 20       10.94          10.83                                                 26       10.93          10.82                                                 30       10.93          10.81                                                 33       10.93          10.81                                                 ______________________________________                                    

During that time at both temperatures listed, the resistance change wasless than 1/1000 of the film's original condition (day=0). The twosamples described in the Table started with different resistance valuesfor the reason that the plates do not have identical dimensions.

Electrochemical corrosion tests were run utilizing a PARC potentiostat,Model 173, to apply a constant potential to either the cathode or anodein the electrochemical cell. This was done by setting the potential ofone of the electrodes relative to a saturated calomel referenceelectrode (SCE). Two tests were run simultaneously in separate cells.One case corresponded to the SOTF used as an anode (positive terminal)with a fixed potential. The counter electrode was a Pt foil. The secondcase has the SOTF situated as the cathode, again using the Pt foil asthe counter electrode.

Shown in Table 2 is the data for ten days of electrochemical tests usingSOTF as the anode.

                  TABLE 2                                                         ______________________________________                                        Potentiostatic corrosion of stannic oxide thin film                           Anode potential = 1.058 V vs S C E                                            Platinum cathode in 5.301 M H.sub.2 SO.sub.4 at 22° C.                 TIME         RESISTANCE                                                       (DAYS)       Ω (20° C.)                                          ______________________________________                                        0            8.12                                                             2            8.11                                                             7            8.11                                                             10           8.12                                                             ______________________________________                                    

With a potential of +1.06 V relative to a calomel electrode, the stannicoxide film did not show a change in resistance within the measurementuncertainty of the experimental apparatus.

The results of using the stannic oxide film as the cathode in theelectrochemical cell are shown in Tables 3, 4 and 5.

                  TABLE 3                                                         ______________________________________                                        Potentiostatic corrosion of stannic oxide thin film                           Cathodic potential - 0.695 V vs S C E                                         Platinum anode in 5.301 M H.sub.2 SO.sub.4 at room temperature                TIME          RESISTANCE                                                      (HRS)         Ω (20° C.)                                                                   R.sub.T /R.sub.o                                    ______________________________________                                        0              7.85       1.00                                                 1/2          10.65       1.35                                                11/2          16.53       2.10                                                ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Potentiostatic corrosion of stannic oxide thin film                           Cathodic potential - 0.1 V vs S.C.E.                                          Platinum anode in 5.301 M H.sub.2 SO.sub.4 at room temperature                TIME          RESISTANCE                                                      (HRS)         Ω (20° C.)                                                                   R.sub.T /R.sub.o                                    ______________________________________                                         0            7.745       1.000                                               66            7.756       1.001                                               90            7.754       1.001                                               130           7.753       1.001                                               ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Potentiostatic corrosion of stannic oxide thin film                           Cathodic potential - 0.350 V vs S C E                                         Platinum anode in 5.301 M H.sub.2 SO.sub.4 at room temperature                TIME          RESISTANCE                                                      (HRS)         Ω (20° C.)                                                                   R.sub.T /R.sub.o                                    ______________________________________                                        0             7.599       1.000                                               1/2           7.622       1.003                                               1             7.641       1.005                                               2             7.667       1.009                                               3             7.678       1.010                                               5             7.868       1.011                                               7             7.696       1.012                                               24            7.863       1.034                                               30            7.933       1.043                                               95            9.589       1.261                                               115           9.981       1.313                                               163           10.873      1.430                                               ______________________________________                                    

It was found that significant deterioration occurs at both -0.70 V and-0.35 V. Reducing the potential to -0.10 V stopped the electrochemicalcorrosion. Over a five day period, there was no measureable change infilm resistance.

After 33 days of conducting chemical corrosion testing, using electricalresistance as the criteria, less than 1/1000 change was detected in themeasurements, i.e., the standard deviation is less than 1/1000. Sincethe error bar in the measurement may be a maximum 2/1000, a conservativeapproach to extrapolating the data is to assume an increase of 2/1000 inthe film resistance every 30 days. At this rate of degradation, the SOTF(stannic oxide thin film) would take 20 years to double the initialelectrical resistance.

The electrochemical corrosion resistance of the SOTF was determined inan electrochemical cell using the SOTF as either the positive ornegative electrode and with Pt foil as the counter electrode. The cellwas set up with a saturated calomel reference electrode (SCE) which wasused to fix the potential of the SOTF electrode. As before, 5.3 Msulfuric acid was used and all electrochemical tests were run at 20° C.The SOTF electrode (coated glass plate) was removed periodically fromthe electrochemical cell and measurements were made of the films. Usingthe SOTF as the anode (positive electrode with a potential of +1.06 Vversus SCE), less than 1/1000 change in electrical resistance wasmeasured after 10 days of continuous running. Given this limited data,it would take approximately seven years for the SOTF to double theinitial resistance value.

Another series of experiments were run using SOTF as the cathode(negative electrode) and Pt foil as the anode at 20° C. nitial runs,where the SOTF potential was set to -1.2 V relative to a SCE referenceelectrode, resulted in a complete degradation or corrosion of the thinfilm within a time frame of five to ten minutes. Running theelectrochemical cell with SOTF at -0.70 V versus SCE and -0.35 V versusSCE resulted in a significant increase in film electrical resistancewith time. For the case of -0.70 V, the resistance doubled with a timeof 1 hour while for -0.35 V the time for doubling of resistance isestimated to be 14 days. Reducing the SOTF potential further to -0.10 Vversus SCE resulted in no noticeable resistance change during five days.Consequently, the threshold potential for degradation of SOTF appears tobe between -0.10 V -0.35 V versus SCE. Polarity reversal below -0.10 Vmust be avoided.

The plate of the invention is a liquid impervious, low resistance,through-plate conductor having application in any stackedelectrochemical cell in which it is desired to provide conduction to anadjacent electrode or an adjacent cell. The plate can be used inbatteries, electrolysis cells, fuel cells, electrophoresis cells, etc.The plate can be used in cells with vertically or horizontally disposedcells. The preferred cell configuration is horizontal since horizontaldisposition of a cell prevents electrolyte stratification and thecontinuous, flat surface of the bipolar plate of the invention willprevent shedding of active electrode material, the most prominentfailure mode for lead-acid cells.

A particular, efficient, horizontal battery configuration is disclosedin my copending application, Ser. No. 279,841, filed July 2, 1981,entitled BIPOLAR SEPARATE CELL BATTERY FOR HIGH OR LOW VOLTAGE, thedisclosure of which is expressly incorporated herein by reference. Inthat application, bipolar plate groupings are placed between monopolarplates to increase available potential voltage. The conductive plate ofthe invention can be utilized as a substrate to form either the bipolarplate or a positive monopolar plate of such a battery. A monopolar platewill differ in having the same polarity material provided on eachsurface thereof, and means to provide lateral conduction to provide forparallel connection to cell groupings.

Referring now to FIG. 6, a biplate groupings 90 can be assembledsurrounding a through-conductive plate 92 of the invention by supportinga layer 94 of positive active lead dioxide material on a first glassscrim sheet 96 and a layer 98 of negative active sponge lead on a secondsheet 100 of glass scrim. These sheets 96, 100 are then placed againstthe plate 92 with the active layers 94, 98 in contact with the surfacesof the plate 92. The scrim sheets are in turn faced with a porous,fibrous mat 104 suitably formed from glass fibers. The porous mat iscapable of releasing any gases formed during operation of the cell andholds the electrolyte. The sheets of scrim fabric 98. 100 may be bondedto the mats 104. By bonding an opposite polarity scrim sheet 106, 108 toeach mat 104, a bipolar grouping can be assembled by alternating layersof plates 92 with bipolar porous mat assemblies 110, 112.

The bipolar groupings can be interspersed with monopolar platesconnected by bus bars to battery terminals. Alternately, the electrodematerials can be plated directly onto the through-conductive substrateplate of the invention. For example, sponge lead can be coated onto onesurface and lead dioxide can be coated directly onto the other surfaceor indirectly onto lead strips coated onto the opposite surface. Bipolargroupings are formed simply by interspersing a porouselectrolyte-separator plate between the active material coated bipolarplate. The active materials can be applied as pastes and cured on thescrim or plate according to state of the art procedures. The activematerials can also be formed in situ according to the state of the artby applying lead to each surface and then placing the electrodematerials in electrolyte and connecting them to a source of potential.

It is to be realized that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

I claim:
 1. A method of forming a through-conductive plate comprisingthe steps of:dispersing conductive tin oxide coated glass in a softenedresin; and forming the resin into a sheet.
 2. A method according toclaim 1 in which the glass is a glass fiber having a diameter from about1 to about 20 microns.
 3. A method according to claim 2 in which the tinoxide is present as a coating having a thickness from a monolayer toabout 10 microns.
 4. A method according to claim 3 in which the coatedfibers are present in the resin in an amount from about 20 to about 80percent by weight.
 5. A method according to claim 4 in which the resinis a polyolefin.
 6. A method according to claim 5 in which the resin ispolypropylene.
 7. A method according to claim 6 in which the sheet ofglass is a sheet of woven fiberglass fabric or a sheet of sinteredglass.
 8. A method according to claim 1 in which said plate has athickness from about 1 to about 20 mils.
 9. A method according to claim1 in which a protective layer selected from thin films of metal and afilm of resin containing a dispersion of conductive particles isdisposed on a surface of the plate.
 10. A method according to claim 9 inwhich the particles are fibers selected from lead or graphite.
 11. Amethod according to claim 1 in which the glass is in the form ofparticles or fibers.
 12. A method according to claim 1 further includingthe step of adhering a conductive film to a surface of the sheet.
 13. Amethod according to claim 12 in which a mixture of fibers in moltenresin is cast onto a surface of said film.
 14. A method according toclaim 1 in which a sheet of tin oxide coated glass is placed adjacent asheet of thermoplastic resin to form an assembly and the assembly isheated and pressed to imbed the sheet of glass into the sheet of resin.15. A method according to claim 14 further including placing a film ofconductive material that is stable in a reducing environment adjacentthe other surface of said sheet of resin before the heating and pressingstep.